This Phase 3 study will evaluate the safety and efficacy of plozasiran injection (ARO-APOC3) in adult participants with severe hypertriglyceridemia (SHTG). After providing informed consent eligible participants will be randomized to receive 4 doses (once every 3 months) of plozasiran or placebo, and be evaluated for efficacy and safety. After Month 12, eligible participants will be offered an opportunity to continue in an optional open-label extension under a separate protocol.

Inclusion Criteria:

• Established diagnosis of severe hypertriglyceridemia (SHTG) and prior documented evidence (medical history) of fasting TG levels of ≥500 mg/dL (≥5.65 mmol/L)
• Mean fasting TG level ≥500 mg/dL (≥5.65 mmol/L) collected at 2 separate and consecutive visits at least 7 days apart and no more than 17 days apart during the screening period
• Fasting low density lipoprotein-cholesterol (LDL-C) ≤130 mg/dL (≤3.37 mmol/L) at screening
• Screening HbA1C ≤9.0%
• Must be on standard of care lipid-lowering medications per local guidelines (unless documented as intolerant as determined by the Investigator, including an inability to safely administer or re-administer a specific drug because of fear, preference, genetic, clinical, or metabolic considerations, or due to a previous adverse reaction associated with, attributed to, or caused by specific drug)

Exclusion Criteria:

• Use of any hepatocyte-targeted small interfering ribonucleic acid (siRNA) that targets lipids and/or triglycerides within 365 days before Day 1 (except inclisiran, which is permitted). Administration of investigational drug and inclisiran must be separated by at least 4 weeks
• Use of any other hepatocyte-targeted siRNA or antisense oligonucleotide molecule within 60 days or within 5-half-lives before Day 1 based on plasma pharmacokinetics (PK), whichever is longer (except inclisiran, which is permitted)
• Known diagnosis of familial chylomicronemia syndrome (FCS) (type 1 Hyperlipoproteinemia) by documentation of confirmed homozygote or double heterozygote for loss-of-function mutations in type 1- causing genes
• Body mass index >45kg/m^2

Note: Additional Inclusion/Exclusion criteria may apply per protocol